1. Field of the Invention
This invention relates to a process for preparing urethanes by reacting a solution of a nitrogen-containing organic compound and a hydroxyl-containing organic compound with carbon monoxide in the presence of a rhodium catalyst.
2 Description of the Art
Various patents have disclosed methods for carbonylating nitrogen-containing organic compounds--e.g., nitro compounds, amines, azo-- and azoxy compounds--to urethanes in the presence of a platinum group metal-containing catalyst, usually a palladium or rhodium-containing catalyst and most often a palladium or rhodium halide-containing catalyst. Generally, a co-catalyst (promoter) has been needed in combination with the platinum group metal-containing catalyst in order to obtain improved rates of reaction. The vast majority of prior art processes use, as a co-catalyst, a halide salt of a metal which is redox-active under the reaction conditions, usually iron, and most often iron chlorides. The co-catalyst is used in substantial molar excess compared to the main catalyst in order to obtain the desired reaction rate. These large quantities of redox-active metal halides are troublesome to separate from the reaction product and cause substantial corrosion problems.
A few references have taught the addition of a primary amino compound (and/or related compounds such as urea, biurets, and allophanates) to further improve the rate and selectivity of reactions catalyzed by a platinum group metal compound in combination with a redox-active metal halide-cocatalyst. U.S. Pat. No. 4,178,455 discloses that, in a process for converting nitroaromatic to urethane catalyzed by a platinum, palladium, rhodium, or ruthenium compound and a Lewis-acid promoter, the rate and selectivity are improved, by adding to the reaction, an organic primary amino compound, a urea compound, a biuret compound, an allophanate compound, or a mixture thereof. The preferred Lewis acid promoters are redox-active metal salts, especially iron chlorides. This patent illustrates (by example) only palladium catalysts with iron chloride promoters. A careful study of the examples reveals that the starting nitroaromatic and the compound) are both converted, in net, to urethane. Thus, when the primary amino compound or urea compound contains the same aryl group as the starting nitroaromatic compound the reported yield of urethane, based on only the nitroaromatic converted, exceeds 100%.
U.S. Pat. No. 4,297,501 discloses a process in which mixtures of a primary amine and a nitroaromatic are carbonylated to urethane with a Group VIII noble metal compound and an oxychloride compound capable of undergoing redox reactions. Alternatively, an oxide compound capable of undergoing redox reactions in combination with a anionic chloride compound is used as the promoter system. Illustrated, by example, are PdCl.sub.2 and RhCl.sub.3, as Group VIII noble metal compounds, and the oxides and chlorides of vanadium and iron. In the preferred embodiment of U.S. Pat. No. 4,297,501, the nitroaromatic corresponds to the primary amine, and the patent teaches the following reaction stoichiometry: EQU 2RNH.sub.2 +RNO.sub.2 +3CO+3R'OH .fwdarw.3RNHCO .sub.2 R'+2H.sub.2 O (1)
U.S. Pat. No. 4,297,501 further teaches that when nitroaromatic is present in excess of the 1:2 ratio relative to amine, the remaining nitroaromatic is converted to urethane by the following reaction stoichiometry: EQU RNO.sub.2 +3CO +R'OH.fwdarw.RNHCO.sub.2 R'+2CO.sub.2 ( 2)
It can be seen from the above equations that when primary amine is initially present, in processes which convert nitroaromatic to urethane using Group VIII noble metals in combination with redox-active metal halide co-catalysts, the amine is, in net, consumed to also make urethane. (See equation (1) above) Once the amine is consumed to low levels, any remaining nitrobenzene is converted to urethane via reaction equation 2) above. Since the amine is already consumed to low levels, it is no longer available to favorably influence the rate of the process according to said reaction (2).
U.S. Pat. No. 4,304,922 similarly discolses a process in which mixtures of N,N'-diaryl urea and nitroaromatic ae carbonylated to urethane with the same catalyst/co-catalyst systems of U.S. Pat. No. 4,297,501. Illustrated by examples are PdCl.sub.2, RhCl.sub.3, IrCl.sub.3, PtCl.sub.4 and RuCl.sub.3 as Group VIII noble metal compounds. Iron oxychloride and several other redox active metal oxides and chlorides are illustrated as co-catalysts. In examples, in which redox active metal oxides are used, anilinium hydrochloride is also added to provide active anionic chloride. In the preferred embodiment of this patent, the N,N'-diaryl urea and nitroaromatic have the same aryl groups, and the patent teaches that the following reaction stoichiometry is obtained: EQU 2RNHCONHR+RNO.sub.2 +3CO+5R'OH.fwdarw.5RNHCO.sub.2 R'+2H.sub.20 O (3)
It is known that N,N'diarylureas react with alcohols to produce urethane plus amine; see for Example U.S. Pat. No. 2,409,712, wherein the following reaction is disclosed: EQU RNHCONHR+R'OH.fwdarw.RNHCO.sub.2 R'+RNH.sub.2 ( 4)
It can be seen that once this occurs under the reaction conditions, the same process, as U.S. Pat. No. 4,297,501 is obtained according to equation (1) above. (Twice equation (4) plus equation (1) equals equation (3)). It can further be seen that both N,N'-diaryl urea and arylamine are, in net, consumed in the proces to make urethane. Example 14 of U.S. Pat. No. 4,304,922 illustrates that when RuCl.sub.3 is used as catalyst in combination with iron oxychloride as co-catalyst, nitrobenzene and N,N'-diphenylurea (1:2 molar ratio) are both consumed (100% and 99% conversion, respectively) to give urethane product (88% selectivity based on nitrobenzene plus N,N'-diphenylurea).
Japan Kokai No. 55-7227 discloses a process in which molecular hydrogen is added, to a process for carbonylating nitroaromatic, in the presence of a palladium catalyst, to increase the reaction rate. The description of the invention specifies a palladium catalyst, accompanied by promoters such as tertiary amines, iron and vanadium compounds, and chlorine ions. All illustrated examples use a supported palladium-selenium on carbon catalyst promoted with pyridine and either FeCl.sub.2 and VOCl.sub.3 (these are redox-active metal chlorides). The patent teaches that the addition of hydrogen causes hydrogenation of a fraction of the nitroaromatic to generate the corresponding arylamine in situ. The process is thus generically similar to that of U.S. Pat. No. 4,178,455, discussed above, which illustrates by example the addition of arylamine to a reaction with a supported palladium catalyst promoted with FeCl.sub.3. Thus, it may be concluded that amine generated from hydrogen will in net be consumed in the reaction to make urethane. Indeed, Japan Kokai No. 55007227 teaches that any amine remaining at the end of a reaction can be returned to another reaction with more nitroaromatic, in which case the amine is easily converted to urethane.
In European patent application No. 86,281 a process is disclosed for converting a nitroaromatic to a urethane in the presence of a primary amine and a catalyst system based on palladium complexed with Group VA-chelate ligands. The application teaches that the redox active metal chloride and related co-catalysts are no longer needed when the above ligands are used. But, this patent also teaches that the amine and/or urea are co-converted with the nitroaromatic to urethane. Thus, the process, in net, consumes added amine or urea. See the examples disclosed in the patent.
Thus, it is clear that, in the processes cited above, as the primary amine and/or urea compound is converted, in net, to urethane, its concentration decreases and its effects on reaction rate and selectivity must also decrease. Eventually, as nitroaromatic continues to be converted, either in a batch process or in a continuous process (with recycle of the remaining amine), the amine will be consumed to a low concentration. In order to maintain the improved rates and selectivities, which are obtained by the original addition of amine, urea, hydrogen, etc., it is necessary to provide additional amine, urea, hydrogen, etc. as the amine is consumed.
A few references teach the use of rhodium catalysts, in the absence of redox-active metal co-catalysts, for the carbonylation of nitrogen-containing organic compounds to urethanes. However, these references do not teach the initial addition of primary amines, ureas, hydrogen, etc. to obtain improved activity. For example, U.S. Pat. No. 3,338,956 discloses a metal carbonyl catalyst of Group VIA, VIIA, or VIIIA for this reaction. The only such catalyst exemplified, however, is rhodium chlorocarbonyl and the rates of reaction are relatively slow.
U.S. Pat. No. 3,993,685 teaches the addition of tertiary amines, especially pyridine, to platinum group metal catalysts to obtain improved activity in the absence of redox-active metal co-catalysts. Rhodium chloride and hydridocarbonyltris(triphenylphosphine) rhodium in combination with pyridine are exemplified.
U.S. Pat. No. 4,052,437 discloses the use of rhodium oxide as catalyst, preferentially in nitrilic solvent. Rh.sub.6 (CO).sub.16 as a catalyst is also exemplified in this patent. There is no suggestion that the initial addition of a primary aryl amine to the process disclosed in this patent would improve the reaction rate.
An article in the Journal of Organic Chemistry 37, (1972) describes a reaction in which nitro-benzene is carbonylated in low yield ( 10%) to urethane with a catalyst comprising Rh.sub.6 (CO).sub.16 in pyridine with ethanol. The major product was aniline.
None of the above cited art, which discloses the use of rhodium catalysts (in the absence of redox-active metal co-catalysts) for the carbonylation of nitroorganics to urethanes, discloses the initial addition of amine, urea, hydrogen, etc. Moreover, the effect of initially adding primary amine to such catalysts is not predictable. Finally, the result obtained by adding a primary amine to a rhodium catalyst system essentially free from redox-active metal components, is substantially different from the result obtained when a primary amine is added to either Group VIII metal catalysts (including rhodium and palladium) in the presence of redox active metal co-catalysts or certain palladium catalysts in the absence of redox active metal co-catalysts.
There are references, which disclose the conversion of nitroaromatic to urethane in the presence of a platinum metal catalysts and in which the amine is not in net consumed. See copending patent applications Ser. Nos. 532,784 and 532,785 both entitled "Process for the Preparation of Urethanes", and filed Sept. 16, 1983, in the names of Grate, Hamm, and Valentine.
It is an object of this invention to provide a process for conversion of nitro-aromatic to urethane in good rate and selectivity, without requiring continual addition of amine, urea, hydrogen, etc. to maintain the rate and selectivity.
It is further object of this invention to effectively carry out the above process in the absence of redox-active metal halide co-catalysts.